Here's a weird little circuit I came up with the other day. I started with a sawtooth frequency doubler designed by Magnus Danielson, and made a few tweaks of my own. The result is a mangler that I have dubbed "The Wave Burper." I made a video of it in action at

It will normally act as a sawtooth frequency doubler, but as you vary the input level, the output will morph from 2X frequency saw to 1X saw to 1X square. You can put a VCA in front of the WB to get voltage control of the wave morphing. However, the real fun starts if you put a VCF in front of the Wave Burper and turn up the resonance a bit. I used a Mankato VCF in the video (of course). I would think of offering a PCB, except it's such a simple circuit that you can knock one out with a bit of perf board in an afternoon (or less!). Have fun!

A couple of notes on the Wave Burper schematic:
1. The vaules in the schematic are for +/-12V operation. For +/-15V, you might want to increase R5 a bit to keep the output at +/-5V. I think I used 75K, but you can experiment to see what works for you.
2. The zener limiters on the output (D1, D2) aren't absolutely necessary, but I found that with some VCFs in front of the WB (especially the Oberheim SEM in Band Pass mode) could produce occasional peaks that went WAY beyond +/-5V. I put the zeners in there to tame those, and I also like the smooth clipping they give when the signal level jumps up.

Thanks for the compliment! You've definitely given me some food for thought there. One thing I was looking at is using another op amp section to make an attenuverter and mix the original (varying levels of inverted or non-inverted) input back with the Burped output. However, feeding the WB with a VCA capable of phase reversal might be spectacular. So... I guess I'm looking for a voltage controlled attenuverter. Anybody got a schematic? Actually, I'm surprised something like this doesn't exist (and maybe I just haven't looked hard enough yet).

Super cool Tim!
i built it with 5.1v zeners and 68k for R5. Looks like it is still in the +-5v range. i used a 1.2k between R1 and the Ground, pretty solid square on the waveform start. Once again, Great unit Tim! Thanks!

How about this for a VCA circuit that would fade between non-inverted and inverted versions of an input?

That looks fine. If you use an OTA where you have "VCA" labeled, then A2 and R6 are not needed! Just feed the OTA (-) input from an appropriate voltage divider. Then design the OTA control current to proved twice the maximum output current as the current through R5, ie 0.1mA max. Finally, drive A3's summing node directly from the OTA output!

This illustrates how it is often advantageous to think of the summing node of an inverting opamp summer as a current summing node, ie, total input current is zero. Then the currents can come from OTA outputs directly as well as from input voltage through input resistors.

D'oh!! Yes, that's supposed to a 1N4734A, 5.6V Zener. This is the 1 Watt version, which is way more power than this diode should ever see, so you could also use the 1/2W version if that's what is available. As noted by Magneticstripper, the 5.1V version seems to work okay too. It probably just begins to clip at a slightly lower level.

How about this for a VCA circuit that would fade between non-inverted and inverted versions of an input?

That looks fine. If you use an OTA where you have "VCA" labeled, then A2 and R6 are not needed! Just feed the OTA (-) input from an appropriate voltage divider. Then design the OTA control current to proved twice the maximum output current as the current through R5, ie 0.1mA max. Finally, drive A3's summing node directly from the OTA output!

This illustrates how it is often advantageous to think of the summing node of an inverting opamp summer as a current summing node, ie, total input current is zero. Then the currents can come from OTA outputs directly as well as from input voltage through input resistors.

Ian

Wow, I definitely would not have thought of that. Nice! Okay, so you could throw this together with a 3080 or half of a 13700 and simplfy the circuit further. I may have to give that a shot this weekend. Thanks for the tip, Ian.

Would it be possible to eliminate A2 by using the OTA's inverting input?_________________"I’m a New Yorker, and I know a con when I see one.” -- Michael BloombergLast edited by richardc64 on Wed May 29, 2013 3:48 pm; edited 1 time in total

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